I. Field
The following description relates generally to wireless communications, and, amongst other things, to flexible communication schemes for wireless communications systems.
II. Background
To enable transmission of data to and from mobile devices, a robust communications network must be enabled. One particular technology utilized in today's mobile networks is Orthogonal Frequency Division Modulation or Orthogonal Frequency Division Multiplexing (OFDM). OFDM modulates digital information onto an analog carrier electromagnetic signal, and is utilized, for example, in the IEEE 802.11a/g WLAN standard. An OFDM baseband signal (e.g., a subband) constitutes a number of orthogonal subcarriers, where each subcarrier is independently modulated by its own data. Benefits of OFDM include ease of filtering noise, ability to vary upstream and downstream speeds (which can be accomplished by way of allocating more or fewer carriers for each purpose), ability to mitigate effects of frequency-selective fading, etc.
Conventional networks must also be able to adapt to new technologies to accommodate an ever-increasing number of users. Thus, it is important to increase a number of dimensions within sectors of a network without substantially affecting quality of data transmission in a negative manner. When utilizing OFDM, increasing dimensions can be problematic as there are a finite number of tones that can be utilized for data communication. Space Division Multiple Access (SDMA) enables an increase in the number of dimensions through sharing of time-frequency resources. For example, a first user and a second user can utilize a substantially similar frequency at a same instance in time in a single sector as long as they are separated by a sufficient spatial distance. Through employment of beams, SDMA can be utilized in an OFDM/OFDMA environment.
In one particular example, beamformed transmissions can be employed to enable SDMA in an OFDM/OFDMA environment. Multiple transmit antennas located at a base station can be used to form beamformed transmissions, which utilize “beams” that typically cover a narrower area than transmissions using a single transmit antenna. However, the signal to interference and noise ratio (SINR) is enhanced within the area covered by the beams. The portions of a sector not covered by a beam may be referred to as a null region. Mobile devices located within the null region will have will have an extremely low SINR, resulting in reduced performance and possible loss of data. Through use of such beams, users separated by sufficient spatial distance can communicate on substantially similar frequencies, thereby increasing a number of dimensions that can be employed within a sector. There may be instances, however, when it is not desirable for a user to employ SDMA. For example, when preceding is desired, or when channel diversity is desired, degraded performance may result with respect to some mobile devices within a particular region.